A cooling system suitable for operation with a two-phase refrigerant is known from DE 10 2006 005 035 B3 and is used for example to cool food which is stored on board a passenger aircraft and intended to be distributed to the passengers. Typically, the food intended for supplying to the passengers is kept in mobile transport containers. These transport containers are filled and precooled outside the aircraft and after loading into the aircraft are placed at appropriate locations in the aircraft passenger cabin, for example in the galleys. In order to guarantee that the food remains fresh up to being distributed to the passengers, in the region of the transport container locations cooling stations are provided, which are supplied with cooling energy from a central refrigerating device and release this cooling energy to the transport containers, in which the food is stored. A cooling system with a central refrigerating device, as opposed to refrigerating units formed separately at the individual transport container locations, has the advantages of a smaller installation volume and a lower weight and moreover entails less outlay for assembly and maintenance. Furthermore, by using a cooling system with a central refrigerating device arranged outside the passenger cabin it is possible to avoid machine noises generated by refrigerating units placed in the region of the transport container locations, which noises are audible in the aircraft passenger cabin and hence may be found to be disturbing.
In the cooling system known from DE 10 2006 005 035 B3, the phase transitions of the refrigerant flowing through the cooling circuit which occur during operation of the system allow the latent heat consumption which then occurs to be utilised for cooling purposes. The refrigerant mass flow needed to provide a desired cooling capacity is therefore markedly lower than for example in a liquid cooling system, in which a one-phase liquid refrigerant is used. Consequently, the cooling system described in DE 10 2006 005 035 B3 may have lower line cross sections than a liquid cooling system with a comparable cooling capacity. What is more, the reduction of the refrigerant mass flow makes it possible to reduce the conveying capacity needed to convey the refrigerant through the cooling circuit of the cooling system. This leads to an increased efficiency of the system because less energy is needed to operate a corresponding conveying device, such as for example a pump, and moreover less additional heat generated by the conveying device during operation of the conveying device has to be removed from the cooling system.
A problem that is posed by the cooling system disclosed in DE 10 2006 005 035 B3 is, however, the high rest pressure of the refrigerant that may arise in the rest state of the system if the refrigerant in the rest state of the system is in the gaseous state. Whereas the rest pressure of the refrigerant in a liquid cooling system, in which for example Galden® is used as a refrigerant, is usually at most approx. 20 bar, in a cooling system designed for two-phase operation, in which for example CO2 is used as a refrigerant, pressures of 170 to 220 bar may arise at a system temperature of approx. 85° C. Consequently, the line system of the cooling system is exposed to considerable loads and has to be designed accordingly.
DE 10 2009 011 797 A1 therefore proposes equipping a cooling system described in DE 10 2006 005 035 B3 with a control valve which is arranged in the cooling circuit of the cooling system and is controlled, upon transfer of the cooling system to its rest state, in such a way that a desired operating pressure arises in the cooling circuit downstream of the control valve. Refrigerant cooled by the refrigerating device is received in a reservoir arranged upstream of the control valve in the cooling circuit.
The operating method described in DE 10 2009 011 797 A1 is able effectively to prevent a region of the cooling circuit lying upstream of the control valve, i.e. the lines and other components, such as for example valves, heat exchangers etc., provided in this region of the cooling circuit, from being subjected in the rest state of the cooling system to the high maximum rest pressure of the two-phase refrigerant that arises if the refrigerant is in the gaseous state. Instead, the region of the cooling circuit lying downstream of the control valve may be kept under a comparatively low operating pressure.
In the cooling systems known from the prior art, there is the problem that it is frequently difficult to accommodate the system components in the very limited installation space available on board an aircraft or even position them relative to one another such that, for example, efficiency-optimised operation of the cooling system is made possible by utilising gravity in the process management.